The field of this invention is control valves, particularly of the type used for mixing abrasive particles with high pressure fluid for abrasive blasting operations.
Abrasive blasting systems have long been and are well known in the art, such as generally disclosed in U.S. Pat. Nos. 3,834,082 and 3,543,444. Typically, such systems require some mechanism for introducing an abrasive from a reservoir into a high pressure fluid line to provide such abrasive particles under high pressure remote from the abrasive reservoir. Some systems utilized a valving structure wherein the flow of sand entering the high pressure fluid source is regulated by a sliding valve adjacent the base of the abrasive reservoir, such as shown in U.S. Pat. No. 3,182,425 or British Pat. No. 1,394,483, or using a gate-type valve as used with the Clemco Model FSV "Sand Miser Control Valve" as manufactured by the Clemco Manufacturing Company, or having a rotatable blocking valve element as shown in U.S. Pat. No. 3,476,440.
In U.S. Pat. No. 3,618,263, a pinchtype valve is used wherein high pressure fluid flow is regulated by adjusting the amount of air flow through the abrasive line rather than the metering of the abrasive by "pinching" the line closed. However, problems with the pinch-type valve systems include difficulty in obtaining complete shut off of abrasive and/or fluid as well as having high abrasive wear adjacent the valve point with a consequent short life span of the abrasive line adjacent thereto.
U.S. Pat. No. 3,924,657 discloses the use of a opposed-piston arrangement mixing valve whereas in U.S. Pat. No. 3,557,498, a rotatable valving plate is incorporated for controlling mixing of the abrasive and high pressure fluid. The prior art as far as known, has further encountered problems due to abrasives plugging up the valving structure with such abrasives becoming lodged therein. Many prior art devices not only require complete system shut downs to cure this defect but also may require complete disassembly of the control valve to remove such foreign particles.
The prior art also includes an abrasive mixing system wherein a closed pressurized tank having abrasive particles therein mixes with high pressure fluid adjacent a nozzle, directing the same upwardly through a stand pipe through the center portion of the abrasive tank with air bleeding outwardly from the stand pipe for further pressurizing the abrasive tank. Pressurized fluid within the stand pipe mixes with the abrasive and the fluid-entrained abrasive is directed outwardly from the top of the pressurized tank for use downstream in abrasive blasting operations. However, such prior art systems require depressurizing the tank, hence the entire system for cut off of the fluid-entrained abrasive flow while further requiring fill-time to repressurize the tank and system. Furthermore, such systems are not practical for multiple user operations as any user can deactivate the entire system by "dumping" the abrasive tank pressure. Further, all operators must have their remote control system in an "on" position for such a unit to operate properly. Additionally, failure of any component of such a system would result in shutting down all operations thereof.